Direct reduction plants for producing direct reduced iron, known as DRI or sponge iron, hot briquetted iron, or the like (in general prereduced materials useful as feedstock for iron and steelmaking), currently produce such materials by contacting a reducing gas, composed principally of hydrogen and carbon monoxide, at temperatures in the range from 750.degree. C. to 1050.degree. C., with a bed of particulate iron-containing material in the form of lumps, pellets or mixtures thereof. The bed of iron-containing material may be static or may be descending by gravity within a reduction reactor. Examples of such processes are described in U.S. Pat. Nos. 3,749,386; 3,764,123; 3,816,101; 4,336,063; 4,428,072; 4,556,417; 5,078,787; 4,046,557; 4,002,422 and 4,375,983.
It is well known that in direct reduction systems the reducing gas and the oxides being reduced reach an equilibrium which does not allow the full utilization of the reducing gas in the reduction reactor. Consequently, for efficiency the currently operating plants recycle regenerated reducing gas in order to minimize the need of make-up reducing gas. However, there has always been the need to purge or otherwise eliminate a significant portion of the spent reducing gas available to be recycled to prevent accumulation of carbon dioxide and inert elements (such as N.sub.2) in the system. The portion of gas purged normally is utilized as fuel in the reformer or gas heater of the system. This utilization as fuel recovers only the heating value of the purged reducing gas but not the chemical value of its costly hydrogen and carbon monoxide. If most of the chemical value, instead of being purged, could rather be utilized for reduction of iron oxides, then the amount of make up gas needed for a given level of production would be lowered, or alternatively the production would be increased for the same reformer capacity. Regeneration to upgrade the spent reducing gas effluent from the reduction reactor involves elimination of the reduction reaction products i.e. carbon dioxide and water which come out of the reduction reactor in the amounts determined by the chemical equilibrium of these products from the residual hydrogen, carbon monoxide, methane (and any other higher hydrocarbons present in minor amounts in the reducing gas).
It has long been known from a number of prior art references, for example U.S. Pat. No. 2,547,685 to Brassert et al; U.S. Pat. No. 4,584,016 to Becerra-Novoa et al.; U.S. Pat. No. 4,001,010 to Kanbara et al.; U.S. Pat. No. 4,129,281 to Ono et al.; U.S. Pat. No. 3,853,538 to Nemeth et al.; and U.S. Pat. No. 4,046,557 to Beggs, to remove water and carbon dioxide from the reducing gas stream which is to be recycled to the reduction reactor. Water is conventionally removed by quench cooling. For CO.sub.2 removal, all these patents, however, teach the utilization of a CO.sub.2 removal unit, usually of the type where the CO.sub.2 -containing gas is contacted with a liquid solution which reacts with said CO.sub.2, and treats the recycle gas as a whole.
These chemical absorption systems have to be provided with some source of heating, normally in the form of steam, for regenerating the solution, which requirement is costly and in some applications is not readily available. The energy needed for regeneration of the CO.sub.2 absorbent solution, and the capital costs for these CO.sub.2 removal units of large capacity are high.
This is to be compared with the CO.sub.2 removal units of the so-called Pressure Swing Adsorption (PSA) and Vacuum Pressure Swing Adsorption (VPSA) systems, which are most preferably used according to the present invention.
The PSA type gas separation systems have also long been known. Exemplary patents are U.S. Pat. Nos. 3,788,037; 4,869,894; 4,614,525; 5,026,406 and 5,152,975. See also U.S. Pat. Nos. 5,833,734 and 5,858,057. These and the other patents cited herein and their content are incorporated by reference.